The present invention relates to a magnetic recording medium, which has very high electromagnetic transfer characteristics and high durability and comprises a magnetic layer where ferromagnetic powder and a binder are dispersed, said magnetic layer being placed on a non-magnetic support member, and in particular to a magnetic recording medium having high durability, in which no head contamination occurs.
The magnetic recording medium is widely used as recording tape, video tape or floppy disk. In the magnetic recording medium, a magnetic layer where ferromagnetic powder is dispersed in a binder is deposited on a non-magnetic support member.
The magnetic recording medium must be at high level in electromagnetic transfer characteristics, running durability and running performance. Specifically, in an audio tape for recording and reproducing musical sound, higher ability to reproduce the original sound is required. In a video tape, high ability to reproduce the original picture and excellent electromagnetic transfer characteristics are required.
In addition to the high electromagnetic transfer characteristics, the magnetic recording medium must have good running durability as described above. To provide good running durability, abrasive material and lubricant are added to the magnetic layer.
In the equipment where the magnetic recording medium is used, the medium is brought into contact with and slides on a magnetic head. As a result, low molecular components in the binder of the magnetic recording medium migrate to the surface of the magnetic layer and are attached on the magnetic head, thereby causing contamination of the magnetic head.
The contamination of the magnetic head causes deterioration of the electromagnetic transfer characteristics. In particular, in an equipment used for high density recording, number of revolutions of magnetic head is high. The number of revolutions of the magnetic head is 9,600 rpm in a digital video tape recorder, 1,800 rpm in an analog video tape recorder for domestic use and 5,000 rpm for video tape recorder for business use. As a result, sliding speed between the magnetic recording medium and the magnetic head increases. Accordingly, high durability is required for the magnetic recording medium.
In order that powder such as ferromagnetic powder or non-magnetic powder is adsorbed well to the binder, a binder containing polar group such as acid group, e.g. sulfonic acid, is used. Also, a compound having acid group such as phenylphosphonic acid, functioning as a dispersant, is used to improve dispersion property of the powder.
For example, JP-B-9507500 discloses a magnetic recording medium, which is obtained by combining ethylene oxide modified phosphoric acid ester with a polyurethane having yield point. In this magnetic recording medium, phosphoric acid ester adsorbs well to ferromagnetic powder, and the ethylene oxide modified alkyl component has high affinity with the binder, which has polar group such as COOH, OH, etc. providing good effect to dispersion property. Further, it discloses a magnetic recording medium, in which the polyurethane has yield point and which provides high dynamic strength and high durability. However, dispersion property and durability of this magnetic recording medium are not high enough to be used as the recording medium of high recording density as currently required.
Further, U.S. Pat. No. 5,318,838 (JP-A-1189025), JP-A-3185621, U.S. Pat. No. 5,176,955 (JP-A-4001917), and U.S. Pat. No. 5,582,910 (JP-A-4263116) teach improvement of dispersion property and durability by using a specific type of organic phosphorus compound to be adsorbed on and to cover surface of the magnetic substance and by dispersing with a binder having polar group such as SO.sub.3 M group. However, because adsorbing ability of the polar group in the binder is weaker than that of the organic phosphorus compound, the binder is not adsorbed very easily, and non-adsorbed binder migrates to the surface of the magnetic layer. Therefore, its durability is not high enough, and there are problems in both dispersion property and durability as a magnetic recording medium of high density recording as required in the current technique.
According to JP-A-4372717, ferromagnetic powder is coated using a dispersant, which has both acid group and basic group, and it is dispersed by a binder which contains polar group. As a result, acid group adsorbs on the surface of ferromagnetic powder, and the basic group goes out and interacts with the binder having acid group or reacts with and bonded to NCO curing agent, thereby improving durability. Because the polar group of the dispersant has both acid group and basic group, there are strong interactions between dispersants in the coating solution and interactions between dispersant and binder. As a result, ferromagnetic powder, dispersant, binder, etc. are dispersed and viscosity of the prepared coating solution tends to increase. Thus, it is difficult to achieve perfect smoothening of the coating surface.
U.S. Pat. No. 5,023,135 (JP-A-1173417), U.S. Pat. No. 5,153,053 (JP-A-3080425), JP-A-3100918, and JP-A-61172213 each describe a binder containing amino group, and a generalized list of dispersants are given where lecithin and phosphoric acid ester are listed. In particular, JP-A-61172213 describes simultaneous use of lecithin (i.e. phosphorus compound) and an amine-modified vinyl chloride binder. However, lecithin has a problem in that it plasticizes the binder and decreases coating film strength, and durability is not high enough as a magnetic recording medium of high density recording as currently required. Such problems are also pointed out in the JP-B-95007500.
It is an object of the present invention to provide a magnetic recording medium which has excellent dispersion property, high smoothness and electromagnetic transfer characteristics. This magnetic recording medium has high running durability, and wearing of the surface of the magnetic layer or head contamination are avoided in repeated running of the magnetic recording medium with high recording density when it is used in digital tape recorder, and it has excellent storage property under high temperature and high humidity conditions.